Acute in Vivo Elevation of Insulin-Like Growth Factor (IGF) Binding Protein-1 Decreases Plasma Free IGF-I and Muscle Protein Synthesis

This study examined whether the acute elevation of IGF-binding protein-1 (IGFBP-1) decreases the plasma free IGF-I concentration and alters in vivo rates of muscle protein synthesis and glucose uptake. The plasma concentration of human IGFBP-1 was increased to approximately 95 ng/ml in conscious catheterized rats infused iv with human IGFBP-1 for 4 h. Infusion of IGFBP-1 also increased the concentration of endogenous (e.g. rat) IGFBP-1 in the blood, and this response was associated with a 2- to 3-fold elevation of IGFBP-1 mRNA in liver and kidney. IGFBP-1 did not significantly alter the plasma concentration of total IGF-I, but decreased circulating free IGF-I levels by about 50%. IGFBP-1 decreased protein synthesis in the predominantly fast-twitch gastrocnemius muscle (20%), and this change resulted from a decreased translational efficiency that was associated with a decreased phosphorylation of S6K1, but not 4E-BP1. Complementary studies demonstrated that IGFBP-1 also decreased the rates of protein synthesis under basal conditions and in response to stimulation by IGF-I when added in vitro to the fast-twitch epitrochlearis muscle. In contrast, IGFBP-1 did not alter in vivo-determined rates of protein synthesis in the slow-twitch soleus muscle, heart, liver, or kidney. The infusion of IGFBP-1 did not significantly alter the plasma glucose or lactate concentration or the whole body rate of glucose production or disposal. The above-mentioned changes were not mediated indirectly by changes in the plasma insulin or corticosterone concentrations, decreased high energy phosphate content in muscle, or hepatoxicity produced by the infused IGFBP-1. These results demonstrate that acute in vivo elevation in IGFBP-1, of the magnitude observed in various catabolic conditions, is capable of selectively decreasing protein synthesis in fast-twitch skeletal muscle and up-regulating the hepatic and renal syntheses of IGFBP-1 per se. Hence, elevations in circulating and tissue levels of IGFBP-1 may be an important mediator for the muscle catabolism observed in various stress conditions.